Synthesis and characterization of square-planar platinum(II) and palladium(II) complexes with pyridine-2,6-dicarboxylic acid (H2dipic). X-ray crystal structure of trans-Na2[Pt(dipic)2]·6H2O and K[Pt(dipic)I]·1/2H2O

The tetra-coordinate complexes trans-[Pt(Hdipic)2]·2H2O (1) and trans-[Pd(Hdipic)2]·2H2O (2) as well as their potassium and sodium derivatives trans-K2[Pt(dipic)2] (3), trans-Na2[Pt(dipic)2]·6H2O (4) and trans-Na2[Pd(dipic)2]·2H2O (5) have been obtained in high yield by reacting aqueous solutions of K2[PtCl4] and K2[PdCl4] with pyridine-2,6-dicarboxylic acid (H2dipic) or its potassium or sodium salts, respectively, in 1:2 molar ratio. 1 and 2 are also formed on reacting equimolar amounts of reagents in water (pH 1.4) leaving in solution unreacted K2[PtCl4] and K2[PdCl4]. The resulting mixtures brought to pH 5 with KOH undergo a redistribution reaction with formation of the monoanionic species K[Pt(dipic)Cl]·H2O (6) and K[Pd(dipic)Cl]·H2O (7), respectively. The iodo derivative K[Pt(dipic)l]·1/2H2O (8) was obtained from the chloro species by replacement of the coordinated chloride with iodide in aqueous solution. All the complexes have been characterized by a number of physico-chemical measurements including the X-ray analysis of trans-Na2[Pt(dipic)2]·6H2O (4) and K[Pt(dipic)I]·1/2H2O (8). Crystals of 4 are monoclinic, space group P21/c, Z=2, with unit-cell dimensions a=11.912(1), b=6.544(1), c=12.699(1) Å, β=91.56(1)°. Crystals of 8 are triclinic, space group P, Z=4, with unit- cell dimensions a=11.018(1), b=11.585(2), c=9.946(1) Å, a=97.99(1), β=115.07(1), γ=73.14(1)°. The structures were solved from three dimensional counter data by Patterson and Fourier methods and refined by full-matrix least-squares to R=0.019 and 0.030 for 1556 and 4118 observed reflections, respectively. The crystal of 4 is built up by centrosymmetric Pt(dipic)22− anions and Na+ cations hexacoordinated by four water molecules and two carboxylate oxygens belonging to the ligands. Each dipicolinate ion behaves as a bidentate ligand, and the resulting coordination around the platinum is square planar. The asymmetric unit of compound 8 contains two independent square planar Pt(dipic)l− anions, in each of them the dipicolinate ion acts as a tridentate ligand and the fourth position is occupied by an iodine atom. The planar complex anions Pt(dipic)l− are stacked but not parallel and Pt atoms form infinite zigzag chains.